This project investigates the neural mechanisms that subserves the monkey's ability to discriminate noxious thermal stimulations. In the present investigation, single neurons were identified in the primary somatosensory cortex (SI) of the anesthesized monkey. Neurons with slowly adapting responses to intense mechanical stimulation received an intradermal injection of capsaicin into the receptive field. Capsaicin is the active ingredient in chili peppers and produces intense pain when injected into the skin of humans. Capsaicin is known to only activate C fibers and therefore is a purely nociceptive stimulus. Neurons that responded to the capsaicin injection were considered to receive nociceptive input. The receptive fields of nociceptive neurons were classified as wide dynamic range (WDR), preferentially responsive to noxious mechanical stimulation or as nociceptive specific (NS) responsive only to noxious stimuli. Nociceptive neurons that responded to intradermal capsaicin were additionally tested for responses to noxious thermal stimulation. A very small percentage of neurons responsive to capsaicin injection responded to noxious thermal stimulation. In past experiments, a response to noxious thermal stimulation was the criterion for a neuron to be classified as nociceptive. These data suggest that nociceptive neurons are common in the somatosensory cortex, contradicting the long held view that ncoiceptive neurons are rare in the primary somatosensory cortex.